Guide to the Basics of Solar Hot Water Heating Systems

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Question: which is most efficient backup system for solar hot water?

The solar system I am pricing has one backup heating element, in the middle of the tank. I expect a lot of my use will be at night, therefore I was an efficient electrical system too. Would the performance of a supply tank with an element on the top and on the bottom, at traditional tank, be more efficient and supply more hot water than the single element, middle mounted heating element? - dan@dudden.com

Reply:

Interesting question Dan. I don't know - insofar as being able to cite an authoritative source without more research. But it may help to understand the role of dual heating elements in electric water heaters. The lower element warms all water in the tank, starting with cold water entering (cold water usually is directed to the tank bottom by a dip tube). The upper element boosts the ending temperature of hot water leaving the tank.

We should both read some detailed descriptions of the operating of dual-element electric water heaters. My experience with models I've tested and repaired was that both elements worked simultaneously.

If we find a single-element electric water heater tank I figure it's probably because it's a pretty small tank.

You may be better served by choosing the tank size closely matched to your needs so as not to heat more hot water than needed, using a timer to turn off the electric heaters when not wanted or needed, and choosing a water heater with a high energy rating.

Question: Thermal slab with pipes deep in the concrete?

Current building regs here in Scotland dictate that I fit a thermal slab of at least 125mm thick as a floor in my house extension. I couldn't see the point of just relying on the sun through the window to heat it so I fitted under floor heating pipes.The slab is 250 mm thick,the pipes are 200mm below the surface,there is 170mm of "kingspan " between the membrane/subsoil and the edges are insulated with 5mm Kingspan.

I intend to fit a solar hot water panel to slowly,maybe over several days ,heat the slab,intending it to be a heatstore rather than a radiator ,say from a boiler system.

I can see from many posts on this site that my pipes are in too deep for a radiant system ,heated from a boiler,but I think that as a heated from below heat store it should provide a very stable source of heat to keep the room warm through the night or dull days( plenty of those in Scotland)
Comments please.

Reply:

Lee, my experience is that pipes deep in the slab don't work well - which is reflected in the documents by experts in the radiant heating indusry. There are two heat transmission issues:

1. heat loss downwards into soil that exceeds the heat flow upwards into occupied space - you MIGHT overcome this by very good insulation between the slab underside and the soil, resulting in a huge heat sink that takes a long time to heat but eventually sends heat upwards; (If this works in your case do let us know.)

2. heat transfer resistance in the upwards direction caused by the thickness of concrete.

In sum, I'm not optimistic. Can you tell me the R-value of 170mm of Kingspan insulation? Also have you insulated the slab perimeter?

For technical details about placing heating tubing of any sort in to concrete slabs, see these articles:

Passive solar heated slabs: SLAB INSULATION, PASSIVE SOLAR

Radiant heat flooring design:

RADIANT HEAT
RADIANT HEAT FLOOR MISTAKES
RADIANT HEAT TEMPERATURES
RADIANT SLAB FLOORING CHOICES
RADIANT SLAB TUBING & FLUID CHOICES

Reader follow-up:

The R value is 5.100 M2.k/w.Yes the edges have 50mm of the same spec kingspan.The floor slab is encosed in the foundation wall and above ground level. Nothing much lost if it doesn't work,just the £70 for the barrier pipe.I will just have to fit a conventional radiator to the wall and connect the panel to the hot water tank if the heat goes down into the subsoil. Do you think that heat would rise through a solid ,as it does through a gas or liquid?

My aim is to help the thermal store by feeding heat in from below, I want the process to be slow( no avoiding that with the pipes in deep eh.)On reflection maybe midway up would have been better,but after the initial outlay for the panels ,the heat is almost free,might just need a few more. Thanks for you comment,will let you know if it works,probably won't be operational for 18 months or so.

Reply:

Lee,

We'd need to discuss details of some of your questions with a heat transfer engineer; but in general, various solids conduct heat at varying rates, and solids won't have internal currents or movements that occur in liquids or gases. Heat transmits through a solid mor or less at the same rate in all directions, with variation of course when one side of a solid is in contact with or exposed to conditions or temperatures different from another side.

In my OPINION, heat transfers down through an uninsulated concrete slab into the (normally cooler) earth at a rate faster than heat will transfer up through an uninsulated concrete slab into air (normally warmer) in a building above.

In the RADIANT HEAT Floor Mistakes to Avoid article cited above, we had to abandone the in-slab radiant heat floor system. Trials confirmed that it was simply impossible to get the building warm - the rate of heat movement into the earth below the slab (not uniformly insulated, not enough insulation, and tubing too deep in the slab) always exceeded heat movement into the building.

We kept the tiny boiler that had been installed to heat this building and considered installing a thin-set radiant heat tubing floor under a new layer of tile atop the original installation. But because we (and the otherwise goofy builder) had done a good job on building insulation of the walls and roof we found that we could heat the building ( a small cabin) with just a couple of electric heaters + some solar gain thorugh windows and skylights) in cold though perhaps not the coldest (northern Minnesota) temperatures.

Your floor will store passive solar gain heat from sun exposure but I suspect the tubing system will operate at a net loss.

Question: is it normal for the solar hot water panels to be empty or to collapse?

Are solar mats supposed to collapse when [the solar heating system] is turned off?

And is [the solar hot water heating system] meant to be and open air system or a closed system? - L.S. 1/22/2014

Reply:

You are asking about solar hot water or solar heating systems that use a water-antifreeze mix. Among the many types of solar collectors are rubber tubes bonded in mats or gangs of tubing fed at either end by a connected rigid tube connected to the inlet and outlet piping for the system.

Depending on the manufacturer, these tub-mats are rather rigid and would look the same with or without water flowing, while others, made of softer plastic or rubber components may thin down when not under water pressure. In both cases the system stays full of water, not air, when the system is activated.

The solar hot water panel ends shown in my photographs just above are installed on a rooftop in Guanajuato, Mexico and are used to provide domestic hot water and to heat a spa or hot tub. This is a rigid mat, it does not "collapse" when water is not circulating. In fact there is so much solar gain on this roof that the owners have blocked off some of the solar collector tubing so that water is not overheated. Not shown in my photos is a pressure relief safety valve installed on the piping to release water in the event of an unsafe or over-pressure condition. You can see the solar water heater piping, a control valve, and the pressure-relief safety valve in this photo.

But in freezing climates and for some heating applications that are heating water for direct use, that is where an antifreeze mix is not being used in the solar heating system, the system is designed to drain-down when off in freezing weather, to prevent freeze-up. A drain-down solar design will indeed be virtually empty of water in that state: additional controls and air valves are installed on such systems to allow air in or to allow air to exit as required to prevent freezing.

I hope that with these details, if your question is not answered fully you can ask again with more information.

...

• Additional technical contributors & reference sources for this article are listed below.
• Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Mr. Cramer serves on the ASHI Home Inspection Standards. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com
• John Cranor is an ASHI member and a home inspector (The House Whisperer) is located in Glen Allen, VA 23060. He is also a contributor to InspectApedia.com in several technical areas such as plumbing and appliances (dryer vents). Contact Mr. Cranor at 804-747-7747 or by Email: johncranor@verizon.net
• Desmex Solar, Blvd. José Ma. Morelos 3649, Col. Purísima de Jerez, León, Gto. C.P.37290 Tel. +52 (477) 788 06 00, Fax. +52 (477) 771 10 02, e-mail: leon@desmexsolar.com: website http://www.desmexsolar.com/ Additional offices in San Miguel de Allende, Guanajuato, Mexico - Carretera a Celaya Km. 1, Plaza Alhóndiga Local 14 (1M), Frente a Mega Comercial Mexicana, San Miguel de Allende, Gto. Tel: +52 (415) 150 73 12 e-mail: sanmiguel@desmexsolar.com, Guadalajara, Mexico - +52 (33) 3165-2454 e-mail: guadalajara@desmexsolar.com, Monterrey Mexico - Tel. +52 (818) 356 43 30 e-mail: monterrey@desmexsolar.com, and Mexico City - Tel. +52 (55) 2643 26 29, Fax. +52 (477) 771 10 02 e-mail: mexico@desmexsolar.com
• Passive Solar Design Handbook Volume I, the Passive Solar Handbook Introduction to Passive Solar Concepts, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v1.pdf
• Passive Solar Design Handbook Volume II, the Passive Solar Handbook Comprehensive Planning Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v2.pdf [This is a large PDF file that can take a while to load]
• Passive Solar Handbook Volume III, the Passive Solar Handbook Programming Guide, in a version used by the U.S. Air Force - online version available at this link and from the USAF also at wbdg.org/ccb/AF/AFH/pshbk_v3.pdf
• The Passive Solar Design and Construction Handbook, Steven Winter Associates (Author), Michael J. Crosbie (Editor), Wiley & Sons, ISBN 978-047118382 or 0471183083 is available at Amazon.com and via the The Passive Solar Design and Construction Handbook, Steven Winter Associates (Author), Michael J. Crosbie (Editor), Wiley & Sons, ISBN 978-047118382 or 0471183083 is available at Amazon.com and via the InspectAPedia Bookstore
• "Passive Solar Home Design", U.S. Department of Energy, describes using a home's windows, walls, and floors to collect and store solar energy for winter heating and also rejecting solar heat in warm weather.
• Solar Water Heating: A Comprehensive Guide to Solar Water and Space Heating Systems (Mother Earth News Wiser Living Series), Bob Ramlow, Benjamin Nusz, ISBN13: 9780865715615
  Heating water with the sun is almost as old as humankind itself, and it is done all over the world. Yet there are strangely few resources on the topic in North America.
  Solar Water Heating fills this gap. It reviews the history of solar water and space heating systems from prehistory to the present, then presents the basics of solar water heating, including an introduction to modern solar energy systems, energy conservation and energy economics. Drawing on the author’s experience as an installer of these systems, the book goes on to cover:
  * Types of solar collectors, solar water and space heating systems and solar pool heating systems, including their advantages and disadvantages * System components, their installation, operation, and maintenance * System sizing and siting * Choosing the appropriate system.
  Since people often get turned off by the up-front cost, the book focuses especially on the financial aspects of solar water or space heating systems, clearly showing that such systems can save significant costs in the long run. Well-illustrated, the book is designed for a wide readership from the curious to the student or professional.
• "Solar Water Heaters", U.S. Department of Energy article on solar domestic water heaters to generate domestic hot water in buildings, explains how solar water heaters work. Solar heat for swimming pools is also discussed.
  • "Heat Exchangers for Solar Water Heating Systems", U.S. DOE describes the types of solar water heater heat exchange methods between the sun and the building's hot water supply
  • "Heat-Transfer Fluids for Solar Water Heating Systems", U.S. DOE, describes the types of fluids selected to transfer heat between the solar collector and the hot water in storage tanks in a building. These include air, water, water with glycol antifreeze mixtures (needed when using solar hot water systems in freezing climates), hydrocarbon oils, and refrigerants or silicones for heat transfer.
  • "Solar Water Heating System Maintenance and Repair", U.S. DOE
  • "Solar Water Heating System Freeze Protection", U.S. DOE,using antifreeze mixture in solar water heaters (or other freeze-resistant heat transfer fluids), as well as piping to permit draining the solar collector and piping system.
  • "Scaling and Corrosion in Solar Water Heating Systems", U.S. DOE
  • www.energysavers.gov/your_home/water_heating/index.cfm/mytopic=12850 is the base U.S. DOE website for these articles
• "Active Solar Heating Systems", U.S. Department of Energy, including
  • "Radiant Heating Systems" U.S. DOE
  • "Absorption Heat Pumps & Coolers", U.S. DOE
  • "Solar Air Heating" U.S. DOE also referred to as "Ventilation Preheating" in which solar systems use air for absorbing and transferring solar energy or heat to a building
  • "Solar Liquid Heating" U.S. DOE, systems using liquid (typically water) in flat plate solar collectors to collect solar energy in the form of heat for transfer into a building for space heating or hot water heating. The term "solar liquid" is used for accuracy, rather than "solar water" because the water may contain an antifreeze or other chemicals.
• "Side by Side Testing of Eight Solar Water Heating Systems", ETSU S/P3/00275/REP/2, DTI Pub URN 01/1292, Contractor, The Energy Monitoring Company Ltd., C Martin, M Watson, 2001
  This study compared the amount of energy that eight modern solar water heating systems could produce over an average year.
  Document: - local copy
  Document: - original is at http://www.berr.gov.uk/files/file16826.pdf

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